American policy-makers tend to view the use of force as a "last resort." President Obama explicitly framed it that way a few months ago in his set-piece speech at the National Defense University, and his predecessors regularly invoked the "last resort" formulation. Indeed, "last resort" is a requirement of the just war tradition, the dominant ethical framework informing states today and is increasingly reflected in international law. For force to be seen as legitimate, it must be seen as a last resort after other peaceful alternatives have been tried and found wanting.

The moral foundations of last resort are well-established and understandable. The use of force always entails some element of horror: the killing of other people — including some irreducible, unintended, but expected killing of innocents — and the destruction of property. In the just war tradition, force is seen as an evil, albeit a necessary one. Therefore, the only legitimate way humans can resort to such evil is if it is the lesser evil among a range of evil choices, which is demonstrated by first trying lesser evils than force.

Last resort is understandable, but not unproblematic. As a policy matter, an earlier-than-last resort use of force might stand a better chance of producing superior outcomes. In Syria, it is likely that an earlier application of American power might have had a higher chance of success than that same force projection would have today. That is one way to understand why Chairman of the Joint Chiefs Gen. Martin Dempsey no longer supports American intervention in Syria.

Also, built into the "last resort" requirement is the need to endure a parade of policy failures, with all of the consequent unintended-but-expected results that policy failure yields. We have been slouching through this parade of failure with regard to Syria over the last two years, and the results are vividly on display.

The reports of a possible major escalation in the use of chemical weapons in Syria introduces a dramatic new twist on policy failure, leading many to speculate that this time the Obama administration actually might act to enforce the red lines the president previously drew. So far, the administration has resolutely signaled the opposite — paying lip service to the idea that "all options are on the table" but otherwise making it clear that the president has no stomach for any level of intervention beyond the provision of light arms (and even that may be more than the administration is willing to execute).

The Obama administration has reversed itself abruptly in previous use-of-force situations. Obama resolutely resisted military options on Libya until France and the United Kingdom forced the issue, at which point he pivoted sharply to embrace the military intervention in Libya — albeit leading it from behind. France seems once again more open to the use of force in Syria, so perhaps the Libyan formula will repeat itself and the Obama administration will find itself leading another military operation in the Middle East, from behind or maybe even more in front.

Making the case in favor of the use of force, the "last resort" criteria of a parade of failure from alternative policies seems well-met. I do not know of an expert who believes that there is an alternative policy likely to produce success.

However, there is another just war criterion that may push against a military intervention: there must be a reasonable prospect of success from the use of force. Here, over a year of messaging from the administration has advanced the opposite claim. President Obama, and some key members of his team, clearly believe that there is no reasonable prospect of success from using force in Syria — at least not as "success" is usually defined. Paradoxically, in justifying their inactivity over the past years, the Obama administration has presented quite an elaborate case against the use of force. Only if success is defined down to mean "delivering a punitive strike for the sake of delivering a punitive strike" is it possible to say that the Obama administration has indicated it thinks force might be "successful."

Indeed, the juxtaposition within the same news cycle of Dempsey’s letter explaining why he thinks there are no good military options and the allegations of a major crossing of Obama’s chemical weapons red-line produced a highly ironic result: it is doubtful that General Dempsey would have written that letter in exactly the same way if the news of the chemical escalation had preceded it, yet it is also the case that most of the military arguments and some of the political arguments he developed would still apply.

In short, if the administration does resort to force now, they will have to overcome two very large hurdles. First, they will have to overcome the enormous residue of failure that waiting for "last resort" to be reached has produced. Second, they will have to overcome all of their public messaging which has talked down the utility of force options in Syria.

Those hurdles may be too high to overcome, especially for an administration with so little policy momentum in the region.